1. Field of the Invention
The present invention relates to a device and a method for adjusting a working point of a superconducting quantum interference device (hereinafter referred to as SQUID). In particular, the invention relates to a device and a method for achieving automatic adjustment of a working point of a SQUID.
2. Description of the Background Art
FIG. 23 is a circuit diagram showing a conventional control circuit for a SQUID with a flux locked loop (hereinafter referred to as FLL). Referring to FIG. 23, the conventional SQUID FLL circuit includes a SQUID 81 having two Josephson junctions formed at predetermined positions and a constant current source 88 supplying constant current to SQUID 81. Magnetic flux to be measured is input from a pickup coil (not shown) to SQUID 81. Voltage output through both ends of SQUID 81 is converted by a transformer 83a, amplified by a preamplifier 83b and then output from an output portion through a multiplier 84 and an integrator 85. A modulation signal at 40 kHz is added by a modulating unit 87 to the output from multiplier 84 for feedback to a field application coil 82 adjacent to SQUID 81. Accordingly, external magnetic flux detected at SQUID 81 is cancelled.
The output of SQUID 81 is input partially to the X axis of an oscilloscope 90 and the output of preamplifier 83b is input to the Y axis of oscilloscope 90.
According to the method above for adjusting the working point of the SQUID by the conventional SQUID control circuit, the amplitude should be adjusted to the maximum by using oscilloscope 90 with a predetermined bias current Ib being applied to the SQUID, and consequently a problem arises of time-consuming adjustment before use. Further, since the adjustment must be made manually, automatic adjustment of the SQUID is impossible.
FIG. 24 is a circuit diagram showing a conventional non-modulation type SQUID control circuit with a flux locked loop (FLL). Referring to FIG. 24, the non-modulation type SQUID control circuit is basically identical to the modulation type control circuit in FIG. 23 except that the former does not include modulating unit 87.
In the initial setting of the non-modulation type SQUID control circuit, a magnetic field bias current is adjusted at an optimum value by watching a waveform on an oscilloscope 90.
According to the method above for adjusting the working point of the SQUID by the conventional SQUID control circuit of non-modulation type, the field bias current applied to a field application coil 82 should be adjusted to allow the amplitude of the waveform of an output signal from the SQUID to attain the maximum by watching oscilloscope 90 with a predetermined bias current Ib applied to the SQUID, and consequently a problem arises of time-consuming adjustment before use. In addition, since the adjustment must be made manually, automatic adjustment of the SQUID is impossible.
Although the SQUID can detect an extremely weak magnetic flux when used for a magnetometer, for example, the SQUID cannot fully fulfill its function due to the influence of noise such as thermal noise caused by an amplifier employed and the like, due to stability concern of circuits such as amplifier, and the like. In order to solve this problem, a circuit according to the synchronous detection system as shown by the block diagram of FIG. 25 has been employed.
Referring to FIG. 25, an output of a SQUID element 91 detecting magnetic flux is input to a lock-in amplifier 93 via a step-up transformer 92. Lock-in amplifier 93 performs synchronized detection based on a synchronizing signal supplied from an oscillator 87 and supplies the result of detection to a DC amplifier 95. The synchronizing signal from oscillator 87 is also superimposed on an output current from DC amplifier 95 and applied to a feedback coil 96. Accordingly, coil 96 generates a magnetic field on which alternating field corresponding to the synchronizing signal is superimposed, this magnetic field is detected by SQUID element 91 and thus a flux locked loop is formed. DC amplifier 95 outputs a signal proportional to the flux density detected by SQUID 91.
A heater 97 is provided in the vicinity of SQUID element 91. Magnetic flux is trapped within a hole of SQUID element 91 when a magnetic field existing in cooling of SQUID element 91 attains a temperature equal to or less than the critical temperature of the superconductor. The trapped flux is released by increasing the temperature by heater 97 to a temperature of at least the critical current. Heater 97 is driven by a power supply circuit 98 and current is supplied to heater 97 as required.
As a conventional heater for releasing magnetic flux, wire dedicated to the heater is processed to form the heater or a resistor having a capacity corresponding to the power supplied to the heater has been employed. Therefore, a resistor having an allowable input greater than 1 W, if 1 W of input power is required for releasing the magnetic flux, or fabrication of a heater having such an allowable input is necessary.
FIG. 26 shows an outside view of a superconducting magnetic sensor using a resistor with an allowable input of 1 W. Referring to FIG. 26, superconducting magnetic sensor 74 includes a SQUID element 75 of about 5 mmxc3x975 mm in size and a resistor 76 of 1 W having a length of about 10 mm that is placed adjacently to SQUID element 75, which are provided on a chip carrier 77 with a diameter of about 30 mm. Resistor 76 occupies a large area on chip carrier 77 as shown in FIG. 26.
The conventional heater employed must have a large allowable input, so that the SQUID cannot be made compact.
One object of the present invention is to provide a device and a method which are easy to use for adjusting a working point of a SQUID.
Another object of the present invention is to provide a device and a method for adjusting a working point of a SQUID which improve precision of SQUID adjustment and enable automatic adjustment.
A further object of the present invention is to provide a device and a method for adjusting a working point of a non-modulation type SQUID which improve precision of SQUID adjustment and enable automatic adjustment.
A further object of the present invention is to provide an easy-to-use SQUID magnetometer capable of easily performing the entire operation including working point adjustment in a short period of time.
A further object of the present invention is to provide a compact heater for a superconducting magnetic sensor.
The objects above are accomplished by a SQUID working point adjustment device including following elements. Specifically, a SQUID working point adjustment device according to one aspect of the invention includes a SQUID, a unit for supplying a bias current to the SQUID, a unit for applying an alternating source at a predetermined frequency to the SQUID to which the bias current is being supplied to generate magnetic field, a unit for picking out from the generated field a signal corresponding to a half period, and a unit for determining an optimum value of the bias current based on the signal corresponding to change of the field.
According to another aspect of the invention, a method of adjusting a working point of a SQUID includes the steps of supplying a bias current to the SQUID, applying an alternating source at a predetermined frequency to the SQUID to which the bias current is being supplied to generate magnetic field, picking out from the generated field magnetic field corresponding to a half period, and determining an optimum value of the bias current based on an output corresponding to change of the picked out field.
The magnetic field is generated at the SQUID to which the bias current is being applied, and the optimum value of the bias current is determined based on an output corresponding to change of the field. Accordingly, the optimum value of the bias current can automatically be determined. Further, precision of the adjustment of the bias current is improved.
According to a further aspect of the invention, a SQUID working point adjustment device includes a SQUID, a unit for supplying a predetermined bias current to the SQUID, and a coil for applying magnetic field to the SQUID. An alternating current at a first frequency and an adjustable direct current are applied to the coil. The SQUID working point adjustment device further includes a unit for picking out a field-voltage characteristic signal corresponding to a half period from the SQUID to which the bias current is being supplied, a filter for passing only a component of the first frequency in the picked out field-voltage characteristic signal, and a monitor for monitoring the signal component passed through the filter.
Preferably, the adjustment device further includes an adjusting unit for adjusting the direct current applied to the coil such that the signal component passed through the filter is at its maximum. At this time, the field sensitivity of the SQUID is maximized.
According to a further aspect of the invention, a method of adjusting a working point of a SQUID includes the steps of applying a predetermined bias current to the SQUID and applying magnetic field to SQUID using a coil. An alternating voltage at a first frequency and an adjustable direct voltage are applied to the coil. The adjusting method further includes the steps of picking out a field-voltage characteristic signal corresponding to a half period from the SQUID to which the bias current is being applied, picking out from the picked out field-voltage characteristic signal only a component of the first frequency, and monitoring the signal component.
The magnetic field is generated at the SQUID to which the bias current is being applied, and an optimum value of the field bias current is determined based on an output corresponding to change of the field, so that the optimum value of the field bias current is easily determined. The bias current is also adjusted more precisely. Further, as the optimum value of the field bias current can be determined, the working point of the SQUID can automatically be adjusted.
A SQUID magnetometer according to the present invention includes a SQUID, an application unit for applying a predetermined bias current to the SQUID, a detector for detecting an output of the SQUID to which the bias current is being applied, a feedback unit for feeding the output of the SQUID back to the SQUID, an application unit for applying a bias field to the feedback unit, and a controller for automatically adjusting the bias field in a state that the bias current is adjusted such that an output voltage of the SQUID is at its maximum.
The bias field is accordingly adjusted automatically in the state that the bias current is adjusted to allow the output voltage of the SQUID to be the maximum one. Manual manipulation of an oscilloscope as conventionally done is thus unnecessary. An easily operable SQUID magnetometer can be provided in this way.
Preferably, the controller includes a data storage unit for storing flux data measured by using the SQUID magnetometer.
Still preferably, the controller further includes an analyzer for analyzing flux data measured by using the SQUID magnetometer.
Still preferably, the SQUID magnetometer further includes a heater for releasing trapped flux, and the controller controls the heater for releasing the trapped flux.
According to a further aspect of the invention, a heater for a superconducting magnetic sensor employing a SQUID element for releasing flux of the superconducting magnetic sensor has a fixed allowable input power. A predetermined input power is applied to the heater for releasing the flux, the allowable input power of the heater being lower than the predetermined applied power.
At a low temperature (about xe2x88x92200xc2x0 C.) which releases the flux of the superconducting magnetic sensor with the SQUID, a rated allowable power at a room temperature has no meaning, and accordingly a resistor having a lower allowable power can be employed. The heater with an allowable input power smaller than the actual input power is used so that the heater and thus the entire device can be made compact.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.